Two new beta-microglobulins were isolated, purified and partially sequenced (rabbit and rat). Antisera against these proteins were produced and the presence of beta sub 2-microglobulins of respective lymphocytes was demonstrated. Antigenic determinants of human and canine beta sub 2- microglobulins were probed by enzymatic methods. A new low molecular weight urinary protein was isolated, purified and its physico-chemical and structural characteristics determined. A radioimmunoassay test was set up to measure the levels of this protein in various biological fluids. Isolation of HL-A antigens was attempted with the aid of immunoabsorbents and large amounts of the proper immunoabsorbents are now being prepared for a similar purpose. Isolation of murine beta sub 2-microglobulin was so far unsuccessful.